Cushing's disease is caused by the pituitary hypersecretion of ACTH and occurs predominately in women. Patients are cured by surgical removal of an ACTH-producing adenoma, suggesting evolution and expansion of a genetically aberrant cell. However, hypothalamic dysfunction and excessive stimulation of anterior pituitary corticotrophs by one or more neurotransmitter substances may also lead to the development of corticotropic adenomas. Allelotyping of the pituitary tumors is being carried out by using restriction fragment length polymorphism (RFLP) analysis. Initial studies showed loss of heterozygosity of genes on chromosome 17 in one of the 4 Nelson's tumors.